1. Field of the Invention
The present invention relates to a multilayer substrate to be used for a hybrid integrated circuit.
2. Description of Related Art
Heretofore, layered green sheet substrates have been produced by (i) printing on green sheets a circuit wiring pattern, (ii) laminating a plurality of printed green sheets and then (iii) sintering the laminated product. For green sheet substrates, layered alumina substrates have been produced by (i) printing on alumina green sheets a circuit wiring pattern of a material composed of a refractory metal, such as tungsten (W) and molybdenum (Mo), (ii) laminating a plurality of printed green sheets and then (iii) sintering the laminated product in a reducing atmosphere (e.g., a mixture of H2 and N2 gases) at around 1,600.degree. C.
Layered glass ceramic substrates have been produced by (i) printing on glass ceramic green sheets a circuit wiring pattern of a material consisting mainly of silver (Ag) or copper (Cu), (ii) laminating a plurality of printed green sheets and then (iii) sintering the layered product in air or in a nitrogen atmosphere at around 800.degree. C. to 1,000.degree. C.
However, the electrical resistivity of tungsten and molybdenum used as the wiring material in the layered alumina substrates is as high as 5.5 .mu..OMEGA..cm or 5.7 .mu..OMEGA..cm, respectively, or three times greater than that of Ag (1.6) or Cu (1.67), conductive materials conventionally used in other layered substrates. Because of this, layered alumina substrates are not usable for applications in fields where the electrical resistivity of wiring must be small. The resistivity of wiring in glass ceramic substrates can be relatively low because of the use of low resistance wiring materials. The substrates, however, are still unsatisfactory when used in applications in the field of, e.g., communications or computers. It has therefore been desired to further lower the resistivity of their wiring.